The present invention relates to electrically commutated motors, and more particularly to controllers and drivers for trapezoidally commutated motors.
Brushless electronically commutated motors have been utilized increasingly in lieu of brushed DC motors in numerous applications, primarily for their advantages of lower cost, higher efficiency, and longer useful life. Electronically commutated motors and drivers generally are provided in two types: sinusoidally commutated motors and trapezoidally commutated motors.
Sinusoidally commutated motors, also known as permanent magnet AC synchronous motors, have a back EMF (electromotive force) waveform similar to a sine wave. In a three-phase motor, all phases are driven simultaneously at different voltages that vary substantially sinusoidally. Separate position sensors, e.g. Hall effect sensors, are required to provide the necessary rotor position information to the electronic drive.
Trapezoidally commutated motors, also known as brushless DC motors, have a somewhat trapezoidally-shaped back EMF waveform. In a three-phase motor, the phases are driven intermittently and in pairs so that at any given time one of the phases is not driven. This allows use of the back EMF signal, in particular its zero crossing, to be used to determine rotor position, a configuration referred to as a sensorless drive.
The motor is driven through selective application of voltages to the different phases in a repeating sequence, i.e. a commutation cycle. FIG. 1 illustrates the six steps of a commutation cycle in a standard brushless DC motor. During each step, two of the phases are in an active state, i.e. either driven at a high voltage or driven at a low voltage, while the third phase is not driven. Between each pair of succeeding steps, two of the phases transition, either from an active state to the inactive state or from the inactive state to one of the active states.
FIG. 2 graphically illustrates the commutation cycle, with motor phases A, B, and C aligned to facilitate recognizing simultaneous transitions. It is to be appreciated that the levels “1,” “−1,” and “0” respectively represent a high voltage, a low voltage (which may be ground), and a center voltage midway between the high and low voltages; in other words, the sum of the high and low voltages divided by two.
FIG. 3 graphically represents the current levels in phases A, B, and C corresponding to the applied voltages indicated in FIG. 2. While each change in voltage between an active state and the inactive state entails a rapid change in current through the particular phase, it further is apparent that a given phase kept constant from one step to the next also experiences a change in current, due to the steep changes in the other phases.
The resultant or sum of the currents in phases A-C is shown in FIG. 4. The repeating fluctuations, known as commutation current ripple, result in a corresponding commutation torque ripple that is undesirable because it increases motor noise and reduces motor efficiency. While this is a tolerable result in many applications, more demanding environments require the more complex and more costly sinusoidal commutation.
Therefore, the present invention has several aspects directed to one or more of the following objects:                to provide a commutation circuit operable to apply different voltages individually to different phases in a manner to reduce commutation torque ripple in a trapezoidally commutated DC motor;        to provide a process for controlling a motor driver to add transitional steps in the commutation cycle to achieve smoother operation;        to provide a controller operable in conjunction with commutation circuitry to apply a more constant resultant or total current through multiple phase windings; and        to provide a more reliable, less costly and longer lasting trapezoidally commutated motor.        